The present invention relates to split housings for an electrical device and, in particular, to split housings employing snap-lock fasteners for timer mechanisms.
Various types of devices are used to fasten together components of electrical apparatus. One type, often called a snap-lock fastener, usually has two or more, preferably three or more, flexible fingers spaced appropriately along the periphery of one of the components to be locked in place. The other component has complementary recesses adapted to receive the fingers. As the components are pushed together, the fingers enter the respective recesses. The fingers initially flex and then snap back into place as the components are nestled together. The components are locked in place by a projection on the free end of each finger which fits snugly in a space behind a catch projection located at the remote end of the recess.
In most electrical applications the fasteners employed are made of plastic material having good electrical insulating properties and flexing properties with requisite strength. The use of injection molding techniques is playing an important manufacturing role as snap-lock fingers are often molded integral with one component and the recess and catch integral with a second.
Examples of various types of snap-lock fasteners may be found in U.S. Pat. Nos. 3,514,554 issued to G. C. Boysen on May 26, 1970, 3,525,825 issued to K. C. Allison on Aug. 25, 1970, and 3,532,840 issued to W. R. Bauer on Oct. 6, 1970. Still other applications for such fasteners can be seen in U.S. Pat. Nos. 3,624,320 issued to Eberhart et al. on Nov. 30, 1971 and 3,866,008 issued to Angelo Toruzzi on Feb. 11, 1975. The manufacture of snap-lock fasteners through an injection molding process is taught in U.S. Pat. No. 4,045,637 issued to R. E. Mongeau on Aug. 30, 1977. Snap-lock fasteners have been used to secure together the split-housings of timer mechanisms as described in U.S. Pat. No. 4,491,710 issued to S. W. Smock on Jan. 1, 1985.
Timer mechanisms typically include electrical contact blades which make and break electrical contact in accordance with a rotational position of a cam. In some applications typified by the aforementioned U.S. Pat. No. 4,491,710, the contact blades, cam and associated components are in one housing while the motor and associated components are in a second housing. Electrical and mechanical communication is made through a separating cover when the housings are secured together. Generally, the contact blades extend through a side wall for appropriate electrical contact by the terminal or free end thereof with the apparatus being controlled.
The various electrical components of the prior art timers described above, particularly those employing split housings, while appropriately secured during normal use, are susceptible to a partial opening due to extraneous forces such as might be developed upon impact from a fall. This is particularly true if the impact occurs upon portions of the electrical components which necessarily protrude from the assembled device such as the free end of electrical contact blade and dials for the manual setting of the timer. The impact upon the free end of the blades can create a torque about the blade causing the undesirable flexing of an adjacent snap-lock fastener arm and permitting the housings and cover to partially separate. While the components may be pushed together to restore the assembly, the unintentionally partial opening provides ample opportunity in the interim for foreign objects to enter into the housings and cause damage to the components.
One solution has been to maintain the arms under tension in the engaged or locked position through structural design of the fastener. While initially minimizing unintentional opening of the housings, the constant tension often relaxes due to the creep characteristics of the plastic material and the fastener reverts to a largely untensioned state which, depending upon the application, can promote rather than prevent partial opening of the housings.
Another disadvantage of the prior art with respect to timer mechanisms are the large sizes and often complex configurations required by the motor housings. The peripheries of the housings generally must conform to each other to facilitate interaction of the fasteners and cooperative recesses. Thus, one of the housings must be constructed to be larger than necessary to enclose the components therein.